The Electronic Sniffer
terminal and the voltage regulator — are soldered to the
top of the board with the
word “Sniff.” The sensor’s six
pins are designed to be
mounted, regardless of its
position; however, the sound
transducer is marked with
polarity. The long pin of the
LED goes to the pad marked R1 R4
+. There are four holes for
the different pin spacings of + +
different sound transducers. 4 D C1
The voltage regulator is -
soldered to the bottom side
of the board, along with the
two-pin terminal. Pin 1 is
marked for the voltage regulator. Take some red fingernail polish and mark the terminal next to the R (red wire)
for ease of wire connection.
To make the bulb adapter, use an extra PR4 bulb and
break the glass. Clean out its base and solder a red wire to
1 2 345
The schematic showing the regulator, sensor, Wheatstone bridge, PIC processor, and output indicators.
the center and a black wire to inside the case. The wires
should be about 1.5 inches long. Tin about 1/4 inch of the
ends. This adapter allows the unit to be connected to the
How the Sensor Works
Most of the Figaro sensors use micro-grains of sintered
tin oxide (SnO2) that have been heated to a high temperature.
This causes oxygen to be absorbed on the grains and provides
a positive potential in the space charge between the molecules. When a reducing gas — such as acetone, alcohol,
propane, etc. — is passed over the sensor, the sensor’s resistance drops. The resistance is linear logarithmically over ranges
of a few parts per million (PPM) to several 1,000 PPM. For
those not familiar with log scales, what it amounts to is that a
large change in PPM will cause a small change in resistance,
e.g., 100 PPM may give a resistance of 2,000 Ω, but a 1,000
PPM would give a resistance of 1,000 Ω. For those who want
the formulations, here they are:
Rs = A[C]-α
where: Rs = electrical resistance of the sensor
A = constant
[C] = gas concentration
α = slope of Rs curve
It is normal for the sensor to have a large drop in
resistance when it is first turned on. Once the heater warms
up, the unit will function normally. The sensors will respond in
a matter of seconds.
Humidity and temperature will also affect the sensor’s
resistance, whereas humidity is difficult to correct for (but not
impossible). I considered it a minor problem due to the nature
of the project. Temperature is compensated for by the
thermistor in the circuit.
Although the sensors are pretty rugged, avoid silicon
vapors and corrosive vapors, such as H2S, Cl2, HCl, and SO.
Don’t get the sensor wet and, if it does get wet, don’t allow it
to freeze or it will crack the sensing material.
Prices vary with the sensors. Here is a short list:
The following models sell for $14.50 and detect these gases:
TGS822 Alcohol, toluene, xylene, acetone, etc.
TGS842 Natural gas/methane
TGS24420* Carbon monoxide
TGS2600* Air contaminants
TGS2610* General combustible gas
* Require different mounting configurations
These models sell for $23.25 and detect:
TGS830 CFC R- 22 and R-113
TGS831 CFC R- 22 and R- 21
TGS832 CFC R- 22 and R-134a
Finally, these models sell for $56.30 and detect:
TGS825 Hydrogen sulfide
Figaro also makes oxygen sensors, but they are rather
bulky and will not fit on the flashlight. Visit them at
www.figarosensor.com for spec sheets and order info.